Semiconductor memory devices are used extensively to store data. Memory devices can be characterized according to two general types: volatile and non-volatile. Volatile memory devices such as static random access memory (SRAM) and dynamic random access memory (DRAM) lose data that is stored therein when power is not continuously supplied thereto.
A DRAM cell without a capacitor has been investigated previously. Such memory eliminates the capacitor used in the conventional 1T/1C memory cell, and thus is easier to scale to smaller feature size. In addition, such memory allows for a smaller cell size compared to the conventional 1T/1C memory cell. Chatterjee et al. have proposed a Taper Isolated DRAM cell concept in “Taper Isolated Dynamic Gain RAM Cell”, P. K. Chatterjee et al., pp. 698-699, International Electron Devices Meeting, 1978 (“Chatterjee-1”), “Circuit Optimization of the Taper Isolated Dynamic Gain RAM Cell for VLSI Memories”, P. K. Chatterjee et al., pp. 22-23, IEEE International Solid-State Circuits Conference, February 1979 (“Chatterjee-2”), and “dRAM Design Using the Taper-Isolated Dynamic RAM Cell”, J. E. Leiss et al., pp. 337-344, IEEE Journal of Solid-State Circuits, vol. SC-17, no. 2, April 1982 (“Leiss”), all of which are hereby incorporated herein, in their entireties, by reference thereto. The holes are stored in a local potential minimum, which looks like a bowling alley, where a potential barrier for stored holes is provided. The channel region of the Taper Isolated DRAM cell contains a deep n-type implant and a shallow p-type implant. As shown in “A Survey of High-Density Dynamic RAM Cell Concepts”, P. K. Chatterjee et al., pp. 827-839, IEEE Transactions on Electron Devices, vol. ED-26, no. 6, June 1979 (“Chatterjee-3”), which is hereby incorporated herein, in its entirety, by reference thereto, the deep n-type implant isolates the shallow p-type implant and connects the n-type source and drain regions.
Terada et al. have proposed a Capacitance Coupling (CC) cell in “A New VLSI Memory Cell Using Capacitance Coupling (CC) Cell”, K. Terada et al., pp. 1319-1324, IEEE Transactions on Electron Devices, vol. ED-31, no. 9, September 1984 (“Terada”), while Erb has proposed Stratified Charge Memory in “Stratified Charge Memory”, D. M. Erb, pp. 24-25, IEEE International Solid-State Circuits Conference, February 1978 (“Erb”), both of which are hereby incorporated herein, in their entireties, by reference thereto.
DRAM based on the electrically floating body effect has been proposed both in silicon-on-insulator (SDI) substrate (see for example “The Multistable Charge-Controlled Memory Effect in SDI Transistors at Low Temperatures”, Tack et al., pp. 1373-1382, IEEE Transactions on Electron Devices, vol. 37, May 1990 (“Tack”), “A Capacitor-less 1T-DRAM Cell”, S. Okhonin et al., pp. 85-87, IEEE Electron Device Letters, vol. 23, no. 2, February 2002 and “Memory Design Using One-Transistor Gain Cell on SOI”, T. Ohsawa et al., pp. 152-153, Tech. Digest, 2002 IEEE International Solid-State Circuits Conference, February 2002, all of which are hereby incorporated herein, in their entireties, by reference thereto) and in bulk silicon (see for example “A one transistor cell on bulk substrate (1T-Bulk) for low-cost and high density eDRAM”, R. Ranica et al., pp. 128-129, Digest of Technical Papers, 2004 Symposium on VLSI Technology, June 2004 (“Ranica-1”), “Scaled 1T-Bulk Devices Built with CMOS 90 nm Technology for Low-Cost eDRAM Applications”, R. Ranica et al., 2005 Symposium on VLSI Technology, Digest of Technical Papers (“Ranica-2”), “Further Insight Into the Physics and Modeling of Floating-Body Capacitorless DRAMs”, A. Villaret et al, pp. 2447-2454, IEEE Transactions on Electron Devices, vol. 52, no. 11, November 2005 (“Villaret”), “Simulation of intrinsic bipolar transistor mechanisms for future capacitor-less eDRAM on bulk substrate”, R. Pulicani et al., pp. 966-969, 2010 17th IEEE International Conference on Electronics, Circuits, and Systems (ICECS) (“Pulicani”), which are hereby incorporated herein, in their entireties, by reference thereto).
Widjaja and Or-Bach describes a bi-stable SRAM cell incorporating a floating body transistor, where more than one stable state exists for each memory cell (for example as described in U.S. Pat. No. 8,130,548 to Widjaja et al., titled “Semiconductor Memory Having Floating Body Transistor and Method of Operating” (“Widjaja-1”), U.S. Pat. No. 8,077,536, “Method of Operating Semiconductor Memory Device with Floating Body Transistor Using Silicon Controlled Rectifier Principle” (“Widjaja-2”), U.S. Patent Application Publication No. 2013/0264656 A1, “Memory Device Having Electrically Floating Body Transistor” (“Widjaja-3”), all of which are hereby incorporated herein, in their entireties, by reference thereto). This bi-stability is achieved due to the applied back bias which causes impact ionization and generates holes to compensate for the charge leakage current and recombination.
Content addressable memories (CAMs) are used in high speed search applications and typically require significant number s of transistors and resources to implement. CAMs are different from typical memory devices in which the user typically supplies an address and the memory device will return the data stored at that address. In a CAM, the user or system will provide the memory device a set of data. The CAM will then search through its contents to see if any data matches the data being provided by the user/system. If matching data can be found, the CAM returns the address(es) upon which the matching data was found.
A CAM typically may consume a significant amount of area since it is a traditional SRAM memory with logic added to implement high speed searching capabilities. A typical CAM cell will include a SRAM memory bit in addition to matching logic required to indicate whether or not this cell has matched the provided data.
A Ternary Content Addressable Memory (TCAM) is a modified Content Addressable Memory which allows it to support an additional “don't care” or “x” state beyond traditional “1” and “0” states supported in other memories including normal CAMs. The “x” state is used as a “don't care.” If this state is selected for a data bit, the compare logic of the TCAM bit should ignore any matching data and always allow this single bit to pass. In addition to storing a “don't care” state within the TCAM memory, the user or system should also have the ability to mask or apply a “don't care” state when applying match data to the TCAM memory. This function is typically implemented by using a normally illegal state of non-complementary data such as “11” or “00” instead of the typically complementary data of “10” or “01”. TCAMs are typically significantly larger that CAM memories since the don't care state is usually stored in a second SRAM cell per TCAM bit. Thus each TCAM cell usually includes 2 SRAM bits, and additional matching logic typically costing a footprint of 16-24 transistors per TCAM cell.
This function is typically implemented by using a normally illegal state of non-complementary data such as “11” or “00” instead of the typically complementary data of “10” or “01”. TCAMs are typically significantly larger that CAM memories since the don't care state is usually stored in a second SRAM cell per TCAM bit. Thus each TCAM cell usually includes 2 SRAM bits, and additional matching logic typically costing a footprint of 16-24 transistors per TCAM cell.
There is a need for content addressable memory that significantly reduces the amount of resources consumed by currently available content addressable memory.
There is a need for content addressable memory that occupies a smaller footprint than currently available content addressable memories.